This invention is related generally to the fields of recombinant DNA technology and genomics. More specifically, it is related to gene targeting vectors and methods for rapidly removing or altering a DNA sequence integrated into mammalian cells or host mammalian organisms, and methods for selecting cells using the methods of the invention.
It is possible to modify a mammalian genome by adding. genetic material, but to further modify the introduced genetic material without causing additional alterations in the remaining genome has been a laborious and time consuming process. A system that would allow the simultaneous deletion of the introduced DNA and/or replacement of the introduced DNA would allow the researcher to monitor both the baseline conditions (deletion state) and any altered states of the inserted DNA in the same genetic background.
The ability to make such site specific alterations, deletions and insertions to transgenic cell lines has been described using various site specific recombinases paired with their DNA recognition sequences, such as Cre-lox or Flp-Frt (S. Fukushige et al., Proc Natl Acad Sci USA (1992)89(17):7905-09; S. O""Gorman, et al., Science (1991) 251:1351-35; B. Sauer et al., Proc Natl Acad Sci USA (1988) 85(14):5166-70; B. Sauer et al., Nuc Acids Res (1989) 17(1):147-61; B. Sauer et al., New Biol (1990) 2(5):441-49. However the methods employed by these workers only allowed one type of change to be made in the introduced DNA, either an insertion or a deletion. Furthermore, the methods employed by these authors to detect and characterize the recombinase formed products are very time consuming and laborious.
We have now invented a vector system and method that facilitates insertion of a query gene into a eukaryotic host cell, and the subsequent removal, insertion, and/or substitution of a different query gene and/or marker gene at the same site within the host cell. One aspect of the invention is a polynucleotide vector, comprising in order of transcription: a regulatable promoter; a first recombinase target site; a second recombinase target site different from said first recombinase target site; a cloning site suitable for insertion of a test gene; an internal ribosome binding site (IRES); an optically-active marker-encoding sequence; a third recombinase target site homologous to either said first recombinase target site or said second recombinase target site. A presently-preferred subgenus is the vector further comprising a second promoter, and a selectable marker operatively associated with said second promoter.
Another aspect of the invention is a method of selecting a host cell having a functioning test gene, comprising: providing a host cell lacking a functioning test gene; inserting into said host cell a vector, said vector comprising a regulatable promoter; a first recombinase target site; a second recombinase target site different from said first recombinase target site; a test gene; an internal ribosome binding site (IRES); a label sequence encoding a detectable marker; and a third recombinase target site homologous to either said first recombinase target site or said second recombinase target site; selecting against cells that failed to incorporate said vector; inducing said regulatable promoter; and selecting for cells that express said detectable marker. A presently preferred sub-genus is the method further comprising: contacting said host cell with a recombinase capable of catalyzing excision of said label sequence.
Another aspect of the invention is the method of altering a host cell comprising a vector of the invention, said method comprising providing a host cell comprising a vector of the invention, and contacting said cell with an effective amount of a recombinase that recognizes said first recombinase target site or said second recombinase target site, such that the portion of the vector between the recombinase target sites is deleted. A presently preferred class of the invention is the method wherein said host cell is contacted with recombinase by intracellular expression of said recombinase. Another aspect of the invention is the method of substituting a query gene and/or marker gene in a host cell, comprising providing a host cell comprising a vector of the invention, and inserting into said cell a polynucleotide comprising a recombinase target site complementary to either said first or second recombinase target site along with an effective amount of a recombinase that recognizes said recombinase target, such that the portion of the vector between the recombinase target sites is replaced with the polynucleotide portion between the two recombinase target sites.